1. Field of the Invention
This invention relates to a process for manufacturing high-purity o-toluic acid from o-xylene. o-Toluic acid is widely used as a raw material for agricultural chemicals, medicines, polymerization initiators, and the like. Depending on its use, 99% by weight or more purity is demanded for o-toluic acid.
2. Description of the Prior Art
o-Toluic acid is produced by oxidizing o-xylene. Normally, a liquid oxidation process (hereinafter referred to from time to time as "liquid-phase air oxidation process") in which o-xylene is oxidized by a molecular oxygen-containing gas using a heavy metal salt of an organic acid such as cobalt naphthenate, cobalt toluate, manganese toluate, or the like as a catalyst. These processes are wholly described in U.S. Pat. No. 2,653,165, U.S. Pat. No. 2,712,549, U.S. Pat. No. 2,712,551, etc.
Japanese Patent Publication No. Sho 52 (1977)-46217 and Japanese Patent Application Laid-open No. Sho 53 (1978)-112831 disclose processes for suppressing side reactions in the above liquid oxidation process. Japanese Patent Application Laid-open No. Sho 56 (1981)-55341 describes a process for the purification of the oxidation product by recrystallization.
The reaction product obtained by the oxidation of o-xylene contains, besides o-toluic acid and water, benzoic acid, phthalide, o-phthalic acid, o-tolualdehyde, o-methylbenzyl alcohol and its ester, and the like as by-products. Thus, the product must be purified by means of solvent extraction, crystallization, distillation, or the like.
Among these processes, distillation is the simplest and easiest process from the aspect of the industrial scale operation. However, separation of o-phthalic acid and phthalide from o-toluic acid by distillation is difficult, because of proximity of their boiling points and formation of azeotropic mixtures. Producing o-toluic acid having a purity of 99% by weight or more in an industrial scale by distillation is unfeasible.
Because of this, in a conventional process of producing high-purity o-toluic acid, the oxidation reaction product of o-xylene is usually, after recovery of unreacted o-xylene therefrom, subjected to a crystallization process for purification, in which crude o-toluic acid is dissolved in a large amount of hot water, followed by cooling to crystallize high-purity o-toluic acid. This purification, however, achieves only a low yield, since a portion of o-toluic acid is dissolved and remained in waste water together with by-products. The o-toluic acid dissolved in the waste water is unrecovered and discharged. Thus, a large amount of waste water which requires activated sludge treatment is formed, and increases the production cost and labor.